Conventional state of the art propulsion systems for large civil aircraft typically include one or more gas turbine engines placed under the wings of the aircraft. However, some studies have indicated that so-called distributed propulsion, which involves having numerous smaller propulsion units preferentially arranged around an aircraft, may provide some significant benefits in terms of noise reduction and fuel efficiency when compared with the current state of the art propulsive arrangements.
One known option for distributed propulsion is to provide numerous propulsive units which are located so as to capture and accelerate slow speed boundary layer air which has formed against the surface of the aircraft. This can lead to a significant reduction in overall fuel burn with the maximum benefit of boundary layer ingestion being achieved when the low speed flow is not mixed with the freestream flow, but is accelerated to homogeneous conditions by the propulsion system.
When implementing boundary layer ingestion, the inlet of the fan intake duct is located flush to a surface of the aircraft so that the low speed boundary layer that has developed can be captured and energized by the propulsion system. Typically, the inlet will be located where the boundary layer air is at its thickest such as towards the trailing edge of the wings or towards the tail end of the fuselage.
Current designs for distributed propulsion and airframe boundary layer ingestion propulsion units have fixed area intakes and outlets. This leads to different performance conditions from the engine which is governed by the specific geometry used and the conditions under which the propulsion unit is being used. It is likely that the geometry would be chosen to maximise fuel efficiency over a flight cycle and would therefore be designed to perform optimally during cruise conditions which dictate the mass flow ingested by the propulsion unit and the speed of the airflow. However, the optimum geometry for cruise may not be well suited to take off, which requires high mass flow ingestion from a low airflow speed, and descent idle, which requires very low airflow ingestions without disrupting the airflow over the wings.
WO2010061071 describes a gas turbine engine which includes an air intake duct which is designed to be aerodynamically continuous with the wing of an aircraft. However, this structure is largely fixed which suffers the lack of variability and the associated problems as described above. In an attempt to overcome this short fall, WO'071 describes the use of a movable flap to adjust the intake area. However, this solution disrupts the airflow over the wing and on the outer surface of the nacelle which can lead to a reduction in performance, rather than an increase as desired due to the movable structure introducing high drag airflows and the like.
The present invention seeks to provide an improved propulsive unit for an aircraft.